Astronomers have witnessed the biggest star explosion ever observed. It occurrred in a distant galaxy, but they say one like it could go off at any moment in our own Milky Way, providing enough light to read by at night. As we hear from VOA's David McAlary, the blast was so massive that astronomers believe it occurred by a different process than the way stars normally end their lives.

No one has ever seen a star explosion, or supernova, like this in all the centuries since the ancient Chinese became the first to record one in the year 185. The event has caused its discoverers to speak in superlatives.

"We have discovered a supernova that stands out as far and away the most powerful, the brightest supernova that has ever been observed," said Nathan Smith.

Nathan Smith is an astrophysicist of the University of California at Berkeley. Here is how his university colleague and co-discoverer David Pooley describes it.

"We have concluded that this is a rare and special event," said David Pooley.

The scientists say the supernova was five times brighter than any seen before. Smith says it has also remained bright longer than any ever known.

"In fact, even after the better part of a year, well after 200 days, it has faded somewhat, but it still is about as bright as a normal supernova at its peak," he said.

The gigantic blast occurred in a galaxy 240 million* light years away, the distance light travels in 240 million years. It was seen in September by ground telescopes and NASA's Chandra X-Ray Observatory.

Most supernovas are explosions of stars eight to 20 times the mass of the sun, but Smith and Pooley estimate that this one was of a star 150 times more massive than the sun, a freakish size they say is as rare as people who are 2.3-meters [7 feet] tall.

They suggest that such an extraordinary star blast must take place by extraordinary means, about which there has been a theory but no examples until now.

Most supernovas occur when massive stars exhaust their nuclear fuel and collapse under their own gravity. The result is either a small, ultra-dense neutron star where a spoonful of material would weigh millions of kilograms, or a much denser black hole, so dense that not even light can escape.

But Smith says this apparently is not what happened with this supernova, named "2006gy."

"Instead of the core of the star collapsing, the core of the star is completely obliterated," he said. "It just blasts away all of its material out into space. So all of these radioactive elements go spewing out to large distances, where they can then heat the material and then glow and we can see the fabulous display that it puts on."

The scientists think a nearby star in our own galaxy seems to be on the verge of such a spectacular light show. No supernovas have been seen in the Milky Way in more than 400 years, but they say a similar huge southern star named Eta Carinae could explode at any time in the next million years.

It is not close enough to threaten our solar system, but David Pooley says the blast would brighten it for a while.

"It would be so bright you could see it during the day and you could read a book by its light at night," he said.

Pooley's and Smith's research will appear in The Astrophysical Journal.